The present invention relates to a process for bleaching pulp and, more particularly, to a process for electrochemically bleaching wood pulp.
Wood pulp, regardless of the process by which it is made, must be bleached if it is to be used in the finer varieties of light colored paper. Ordinarily, wood pulp has a portion of the lignin present in the raw fiber associated with the cellulose. Lignin contains certain colored bodies of a highly complex chemical composition which cannot be removed by any amount of washing or mechanical treatment. Thus, the purpose of all successful bleaching practices in the paper industry is to thoroughly bleach the pulp so as to produce maximum whiteness with a minimal deleterious effect on the physical and chemical properties of the pulp.
Originally, wood pulp bleaching was performed in a single operation. More recently, multistage bleaching has become the common practice. Multistage bleaching involves the addition of bleaching chemicals in stages, which are separated by intermediate washing stages with water or alkali and removal of the reaction products. In general, wood pulps are bleached with chlorine compounds, either direct chlorine plus hypochlorite, hypochlorite alone, or chlorine dioxide. One of the more common multistage bleaching processes is a three-stage process comprising: (i) chlorination, (ii) extraction, and (iii) hypochlorite bleaching.
Electrochemical bleaching of wood pulp is known in the art. In these processes, the pulp is bleached by chlorine generated through the electrolysis of a chloride salt. Sodium chloride is used since it is most abundant. Soviet Pat. No. 555,190 teaches one method in which electrolysis is performed at a 6% electrolyte concentration, an initial pH of 2.0 to 2.5, and an anode current density of 0.20 to 0.24 A/cm.sup.2. Because hydrogen is evolved through the reduction of hydrogen ion during the electrolysis, and because sodium is generated forming sodium hydroxide with water, the pH of the solution gradually increases from the initial pH of 2.0-2.5 to a final pH of about 12.0-12.3 and the pulp is subjected to treatment at the intermediate pH.
Nassar, Fadaly, and Sedahmed, "A New Electrochemical Technique for Bleaching Cellulose Pulp," J. Applied Electrochemistry, Vol. 13, p. 663 (1983), examine the process of electrochemical bleaching. Nassar et al. identify a series of reactions according to which chlorine, hypochlorous acid, hypochlorite, and chlorate exist in the bleaching liquor. Nassar et al. also studied the electrochemical system at neutral and alkaline pH.
Electrochemical bleaching is advantageous because the cost of the bleaching process is potentially reduced, and the hazards involved in chlorine transportation and handling are eliminated. In chlorine bleaching, the pulp mills usually have to buy the required chlorine from chlorine distribution centers in liquid form, under high pressure, and at a high cost. In addition, electrochemical bleaching eliminates the pulp mills' dependence on chlorine peoducers. Thus, by installing an electrochemical bleacher, the pulp mills avoid work interruption because of strikes or shutdowns of chlorine plants.